This invention relates generally to the reduction of compressor noise. One possible application of the system is for gas turbine engines, and in particular auxiliary power units.
Auxiliary power units (APUs) were originally designed to meet aviation power needs during ground operations, when the main engines are not running. APUs provide power for electrical and instrumentation systems, hydraulic systems, and main engine startup, and supply cabin air to the environmental control system. Increasingly, APUs are also configured for in-flight functionality, both as a standalone source of accessory power and cabin air, independent of the main engines, and as an emergency backup in the event of main engine failure.
APUs for commercial and military aircraft are typically designed around a gas turbine engine. The gas turbine engine includes a compressor, a combustor and a turbine, arranged in flow series. The compressor and turbine are rotary devices, each with a number of blades oriented radially around a rotor hub or spinner. The compressor supercharges the combustor and, in some configurations, provides pressurized air for the environmental control system and/or various pneumatic accessories. The combustor ignites a fuel-air mixture to produce hot combustion gases, which drive the turbine. The turbine drives the compressor, and delivers rotational energy to an electrical generator, pumps, or other mechanical accessories.
Gas turbine engine compressors rotate at high speeds, and in some designs the blade tips approach supersonic velocity. The result is a series of shock waves generated at the blade passing frequency (BPF), where the BPF is a “pure tone” frequency at which compressor blades pass a given fixed point in space, and which exceeds the broadband noise portion of the acoustic spectrum. As shock waves propagate from the near field at the compressor face into the far field inside the inlet duct, they degenerate into a multi-tone sound spectrum characterized as “buzz saw” noise. The multiple tones occur at engine shaft harmonic frequencies, representing a redistribution (or shift) of acoustic energy away from the single BPF frequency into multiple frequency tones. The resulting sound quality has a characteristically annoying sound quality, and buzz saw noise can be an environmental concern. Turbine inlet silencers have been developed to reduce this component of compressor noise, but there remains an ongoing need for new techniques that complement and enhance this approach.